The different ways presently known for obtaining divalent organolanthanides comprise:
either the reaction of an organometallic compound of the main series, such as an organosodium, an organolithium, an organomagnesium or an organoaluminum compound, with a lanthanide divalent salt such as a chloride, bromide or carboxylate;
or a transmetallation between the lanthanide metal, for example ytterbium, and an organomercuric compound, such as dicyclopentadienyl mercury;
or the dissolution of the lanthanide metal, such as europium or ytterbium for example, in liquid ammonia, followed by a reaction with a hydrocarbon containing an acid hydrogen (pKa&lt;20) such as cyclopentadiene;
or the co-condensation, at low temperature, for example -196.degree. C., of metal vapor and of an unsaturated substrate.
The first two methods require the use of an organometallic compound obtained by other ways and then the separation of the by-products; the first two techniques are complex, the method with liquid ammonia giving amminated by-products and the vaporization method being limited by the metal amounts practically possible to vaporize. In addition, each of these methods leads only to the specific corresponding compounds.